The invention relates to an apparatus for setting the tuning voltage in tunable resonant circuits, particularly in radio receivers.
In radio receivers, frequency-determining components of resonant circuits and other frequency-selective circuit configurations are tuned to a desired frequency or to a desired frequency range.
In integrated radio receivers, the resonant circuits are set by varying the bias voltage of variable-capacitance diodes or varactor diodes, whose capacitance decreases as the tuning voltage increases. In contrast to discrete components, such as tuning capacitors, such variable-capacitance diodes have the advantage that their Technology can be integrated on a semiconductor chip during manufacture, and they are therefore more economical to manufacture and, at the same time, miniaturization of the receiver is made easier.
In radio receivers, various programs can be received, amplified and reproduced on different frequency bands. Tuning circuits are used to set a desired frequency in order to ensure reception of a particular received signal transmitted on this frequency. To this end, an oscillator circuit is used to set an oscillator frequency, which is shifted by a fixed, predetermined intermediate frequency to produce the desired reception frequency, and is supplied to a mixer. The oscillator frequency and the prefiltered received signal, which is also supplied to the mixer, are used to produce the intermediate frequency signal. The mixer therefore has preliminary and intermediate resonant circuits connected upstream of it, which are used as frequency filters for the reception frequencies.
Tuning of the resonant circuits is not carried out manually, as was previously customary, but instead, in more recent receivers based on the prior art, is performed under electronic control.
A conventional receiver whose resonant circuits are tuned using an electronic control is shown in FIG. 1. For this purpose, the receiver has an antenna A which receives a radio signal and outputs it via a line to a first resonant circuit, the so-called preliminary circuit. The received signal is filtered by the preliminary circuit VK on the basis of the reception frequency and is then output to an amplifier V. The amplifier V amplifies the filtered received signal, which is again filtered on the basis of the reception frequency by a second, downstream resonant circuit, the so-called intermediate circuit ZK. The received signal filtered by the preliminary circuit VK and the intermediate circuit ZK is passed on to a mixing device M which filters the filtered signals onto a desired frequency range as a result of a voltage-controlled oscillator resonant circuit VCO setting an oscillator resonant frequency equivalent to the desired reception frequency. The desired intermediate frequency ZF at the output of the mixer M is 10.7 MHz, for example. The intermediate frequency ZF is given as the difference between the reception frequency fE and the oscillator resonant frequency fVCO.
fZF=fVCOxe2x88x92fE
In a typical FM receiver, the reception frequency fE is in the range between 87.5 MHz and 108 MHz. Accordingly, the oscillator resonant frequency of the voltage-controlled oscillator VCO is between 98.2 MHz and 118.7 MHz, that is to say increased by the intermediate frequency fZF of 10.7 MHz.
The oscillator resonant frequency fVCO is set through the use of an oscillator tuning voltage VT which can be regulated.
The output signal from the voltage-controlled oscillator VCO is supplied via a feedback line to a phase locked loop PLL, which produces the oscillator tuning voltage VT. As the oscillator tuning voltage VT rises, the oscillator resonant frequency fVCO increases, as can be seen from the graph in FIG. 2. The frequency spacing xcex94f between the resonant frequency fVCO of the oscillator resonant circuit VCO and the reception frequency fE is ideally exactly the same size as the intermediate frequency fZF, for example 10.7 MHz. Ideally, the two curves fVCO and fE run parallel over the whole frequency range, i.e. the resonant circuits VK and ZK should ideally be set so that the frequency curve fE always runs parallel to the oscillator resonant frequency fVCO, offset by the intermediate frequency fZF. However, theoretical considerations and component tolerances mean that such an ideal parallel curve shape, which is also called ideal synchronism, cannot be achieved.
In known receivers, the tuning circuits are iteratively adjusted in an attempt to approximate to ideal synchronism S by calculating linear coefficients for amplifying the oscillator tuning voltage VT.
For this purpose, the oscillator tuning voltage VT is supplied to a first linear amplifier circuit V1 and to a second linear amplifier circuit V2 for the purpose of tuning the preliminary circuit VK and the intermediate circuit ZK.
In this case, the tuning voltage VTVK for the preliminary circuit is produced on the basis of the following equation:
VTVK=Y1xc2x7VT+X1
The tuning voltage VTZK for the intermediate circuit is calculated as follows:
VTZK=Y2xc2x7VT+X2
The multiplication coefficient Y and the addition coefficient X are determined and stored once, during manufacture or when turning on the receiver, as a result of a maximum adjustment of the output voltage of the mixer.
FIG. 3 shows the capacitance curve for a variable-capacitance diode in a tunable resonant circuit as a function of the applied tuning voltage VT. The variable-capacitance diode or variable-capacitance varactor diode is a reverse-biased semiconductor diode having a hyperabrupt pn-junction or a metal-semiconductor junction, wherein the voltage dependency of the depletion-layer capacitance is utilized. As can be seen from FIG. 3, the capacitance of the varactor diode decreases nonlinearly as the tuning voltage increases. The variable-capacitance varactor diode is more sensitive at a low tuning voltage VT than at a high tuning voltage. With a voltage change xcex94U, the change in capacitance xcex94C1 is larger than the capacitance change xcex94C2 at a higher tuning voltage.
In conventional setting apparatuses, the tuning voltage for the preliminary circuit VK, for example, is linearly dependent on the tuning voltage VT.
FIG. 4 shows the dependency of the capacitance of the varactor diode on the oscillator tuning voltage VT. As can be seen from the bottom graph in FIG. 4, the tuning voltage VTVK, produced by the amplifier setting device V1, for the preliminary circuit VK falls linearly as the tuning voltage VT increases, so that a voltage change xcex94VT1 results in a capacitance change xcex94C1, and a voltage change xcex94VT2 results in a capacitance change xcex94C2. If the voltage change xcex94VT2 is the same as the voltage change xcex94VT1, FIG. 4 shows that the capacitance change xcex94C1 at a high tuning voltage VT is significantly larger than the capacitance change xcex94C2 at a low tuning voltage VT. Since the tuning voltage VT is set digitally by the microprocessor xcexcP, the smallest voltage change xcex94VT is equivalent to one bit. As FIG. 4 shows, the change in the microprocessor""s control signal by the smallest unit, i.e. by one bit, produces different capacitance changes, and hence frequency changes, in the tuning circuits, depending on what point is taken on the linear amplifier curve. In the linear tuning method shown in FIG. 4, the nonlinear capacitance curve for the varactor diode results in falsifications, distortions or corruptions, because the signal resolutions of the control signal are constant over the whole amplification range.
The nonlinearity of the tuning component within the tunable resonant circuit therefore produces corruptions when tuning the resonant circuits, which impairs synchronism.
This problem exists in all resonant circuits adjusted by a tuning component whose setting variable has a nonlinear curve.
It is accordingly an object of the invention to provide a setting apparatus and a setting method for setting tunable resonant circuits which overcome the above-mentioned disadvantages of the heretofore-known apparatusses and methods of this general type and which compensate for nonlinearities in the tuning component.
With the foregoing and other objects in view there is provided, in accordance with the invention, in combination with a tunable resonant circuit connected to a phase locked loop, a setting apparatus for setting a tuning voltage for the tunable resonant circuit. The setting apparatus includes:
an amplification device receiving a digital gain control signal for setting a gain, the amplification device having a signal resolution differing as a function of the gain;
the amplification device generating the tuning voltage by nonlinearly amplifying an oscillator tuning voltage being output by the phase locked loop;
the amplification device having a first digital/analog converter and a second digital/analog converter, the first and second digital/analog converters respectively having an input for receiving analog voltages and respectively being controllable by a digital control signal;
the input of the first digital/analog converter being supplied with an analog voltage in dependence of the oscillator tuning voltage; and
the input of the second digital/analog converter being supplied with a further analog voltage provided by a constant signal.
In other words, the setting apparatus according to the invention is provided with an amplifier device which amplifies an oscillator tuning voltage, output by a phase locked loop, nonlinearly to produce a tuning voltage, in which case the gain can be set by a digital gain control signal and the signal resolution increases as the gain decreases.
In accordance with another feature of the invention, the amplification device is configured such that, as a function of the digital gain control signal, the signal resolution increases as the gain decreases.
In a further advantageous embodiment of the setting apparatus according to the invention, an offset apparatus is provided which amplifies a reference voltage linearly to produce an offset voltage.
In one advantageous embodiment of the setting apparatus according to the invention, the tuning voltage can be displaced or shifted by the offset voltage.
In a further advantageous embodiment of the setting apparatus according to the invention, a summation device is provided which sums the offset voltage and the tuning voltage to produce a tuning summed voltage, which is used to tune the tunable resonant circuit.
In a further advantageous embodiment of the setting apparatus according to the invention, the digital cain control signal and the digital offset control signal are produced by a microprocessor.
In a further advantageous embodiment of the setting apparatus according to the invention, the microprocessor receives a measurement signal from a signal measuring device.
In a further advantageous embodiment of the setting apparatus according to the invention, the signal measuring device records an output signal amplitude of a mixer connected downstream of the tunable resonant circuits.
In a further advantageous embodiment of the setting apparatus according to the invention, the microprocessor sets the digital gain control signal and the digital offset control signal such that the amplitude of the output signal from the mixer attains a maximum.
In a further advantageous embodiment of the setting apparatus according to the invention, the oscillator tuning voltage is converted by a voltage/current converter into a tuning current which is amplified by a current amplification device.
In a further advantageous embodiment of the setting apparatus according to the invention, the reference voltage is converted by a voltage/current converter into a constant current which is amplified by a current amplification device.
In a further advantageous embodiment of the setting apparatus according to the invention, the amplified tuning current and the amplified constant current are summed at a summed current node to produce a summed current.
In a further advantageous embodiment of the setting apparatus according to the invention, the summed current is converted by a current/voltage converter to produce the tuning summed voltage.
In a further advantageous embodiment of the setting apparatus according to the invention, the tuning voltage sets the capacitance of a variable-capacitance varactor diode in the tunable resonant circuit.
A tuning method for tuning circuits has the following steps, specifically
an oscillator tuning voltage is produced by a phase locked loop,
the oscillator tuning voltage is amplified by an amplifier device with nonlinear gain,
the tuning circuits are tuned using the amplified oscillator tuning voltage,
the output signal from the tuning circuit is mixed with an oscillator mixed frequency signal,
the output signal amplitude of the mixed signal produced by the mixer is measured,
the gain is varied by a control device until the measured output signal amplitude attains a maximum.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an apparatus for setting the tuning voltage of tunable resonant circuits, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.